Mitigation of yield-scaled greenhouse gas emissions from irrigated rice through Azolla, Blue-green algae, and plant growth-promoting bacteria

Irrigated transplanted flooded rice is a major source of methane (CH4) emission. We carried out experiments for 2 years in irrigated flooded rice to study if interventions like methane-utilizing bacteria, Blue-green algae (BGA), and Azolla could mitigate the emission of CH4 and nitrous oxide (N2O) and lower the yield-scaled global warming potential (GWP). The experiment included nine treatments: T-1 (120 kg N ha(-1) urea), T-2 (90 kg N ha(-1) urea + 30 kg N ha(-1) fresh Azolla), T-3 (90 kg N ha(-1) urea + 30 kg N ha(-1) Blue-green algae (BGA), T-4 (60 kg N ha(-1) urea + 30 kg N ha(-1) BGA + 30 kg N ha(-1)Azolla, T-5 (120 kg N ha(-1) urea + Hyphomicrobium facile MaAL69), T-6 (120 kg N ha(-1) by urea + Burkholderia vietnamiensis AAAr40), T-7 (120 kg N ha(-1) by urea + Methylobacteruim oryzae MNL7), T-8 (120 kg N ha(-1) urea + combination of Burkholderia AAAr40, Hyphomicrobium facile MaAL69, Methylobacteruim oryzae MNL7), and T-9 (no N fertilizer). Maximum decrease in cumulative CH4 emission was observed with the application of Methylobacteruim oryzae MNL7 in T-7 (19.9%), followed by Azolla + BGA in T-4 (13.2%) as compared to T-1 control. N2O emissions were not significantly affected by the application of CH4-oxidizing bacteria. However, significantly lower (P<0.01) cumulative N2O emissions was observed in T-4 (40.7%) among the fertilized treatments. Highest yields were observed in Azolla treatment T-2 with 25% less urea N application. The reduction in yield-scaled GWP was at par in T-4 (Azolla and BGA) and T-7 (Methylobacteruim oryzae MNL7) treatments and reduced by 27.4% and 15.2% in T-4 and T-7, respectively, as compared to the T-1 (control). K-means clustering analysis showed that the application of Methylobacteruim oryzae MNL7, Azolla, and Azolla + BGA can be an effective mitigation option to reduce the global warming potential while increasing the yield.